(a) Field of the Invention
The present invention relates to a method and apparatus for acquiring synchronization of a terminal in a cooperative communication network.
(b) Description of the Related Art
A cooperative communication network is a concept that was proposed to solve the problems of signal attenuation caused by multi-path transmission in wireless communication and packet loss cooperation caused by channel error, and to maximize bandwidth efficiency. Cooperative communication allows two or more wireless nodes having a single antenna to transmit data to their destination through multiple paths formed by sharing their resources. Moreover, with the use of cooperative communication, the signal transmission/reception efficiency of a terminal located at a cell boundary can be increased by expanding the coverage of a base station.
In a cooperative communication environment, a plurality of base stations interconnected through a hub node transmit signals simultaneously via downlink under control of the hub node. A terminal at a cell boundary uses preamble signals, which are transmitted via downlink from a plurality of base stations in the vicinity, to acquire downlink synchronization with the base stations. Hereupon, the terminal can acquire information about the conditions of wireless paths to the base stations from the preamble signals.
Proper data transmission timing ensures that all systems interpret the start of the information transfer correctly. A preamble defines a specific series of transmission pulses that is understood by communicating systems to mean “someone is about to transmit data”. This ensures that systems receiving the information correctly interpret when the data transmission starts. The actual pulses used as a preamble vary depending on the network communication technology in use.
Respective preamble signals transmitted by the plurality of base stations arrive at the terminal at different times due to different path delays, and the terminal acquires time synchronization for a signal with higher strength or better signal to noise ratio (hereinafter, ‘SNR’), among the received preamble signals.
Typically, a signal that arrives at the terminal earlier shows high strength or good SNR. However, a signal that arrives at the terminal later may show higher strength or better SNR when the base stations connected to the hub node are not synchronized, or when signals pass through a relay, or when there is difference between paths through which signals are reflected and arrive at the terminal.
Hereupon, the terminal can acquire time synchronization based on the arrival timing of a preamble signal received later. In this case, a preamble signal that arrives earlier may cause inter-symbol interference (hereinafter, ‘ISI’) in a demodulation process.
In orthogonal frequency division multiplexing (hereinafter, ‘OFDM’), which allows high-speed data to be transmitted in parallel at a low speed, ISI arises when the delay spread time of a data transmission channel is shorter than a symbol duration. In OFDM, fast Fourier transform (hereinafter, ‘FFT’) is performed so as to demodulate signals received via uplink and downlink. ISI may occur if the start of an FFT execution period is affected by the preceding symbol within the protection period of OFDM symbols.
That is, ISI may occur because an FFT execution period of a signal frame is not included in a period containing a symbol and a cyclic prefix (hereinafter, ‘cp’) due to delay spread time.
ISI may lower the demodulation performance of a signal and reduce SNR, and therefore it is important to acquire time synchronization of uplink and downlink between the terminal and the base stations to prevent ISI.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.